Exhaust emission control devices may comprise catalytic converters, evaporative emissions devices, scrubbing devices (e.g., hydrocarbon, sulfur, and the like), particulate filters, traps, adsorbers, absorbers, non-thermal plasma reactors, and the like, as well as combinations comprising at least one of the foregoing devices. One function of these devices is to treat an exhaust gas, thereby reducing the concentration of at least one component in the gas. Such devices may be rated in terms of their performance, wherein the performance of an exhaust emission control device represents a measure of the ability of that device to reduce the concentration of a component(s) in a gas under various conditions.
Catalytic converters are one type of an exhaust emission control device, and comprise one or more catalytic materials disposed on a substrate. The composition of the catalytic materials, the composition of the substrate, and the method by which the catalytic material is disposed on the substrate serve as one way in which catalytic converters are differentiated from one another. Methods of disposing a catalytic material onto a substrate include washcoating, imbibing, impregnating, physisorbing, chemisorbing, precipitating, and combinations comprising at least one of the foregoing deposition methods.
Washcoating includes contacting an admixture of various components of a catalyst with a substrate such that a layer is disposed on and/or in the substrate. The term washcoat as used herein describes the layer or layers of the catalytically active admixture disposed on the substrate. In a three-way conversion catalyst, the washcoat may comprise a metal component, an aluminum oxide component, and an oxygen storage component. Stabilizers, binders, hydrogen sulfide control agents, and the like may also be present in a washcoat. Furthermore, the washcoat may comprise one or more layers of material.
The various components in a washcoat may serve more than one function. For example, the aluminum oxide component (aluminum oxide) in a three way conversion catalyst may act as a support for other components, it may act as a binder within the washcoat or between the washcoat and the substrate, and/or provide a thermally stable porous diluent in the washcoat. The aluminum oxide may also provide catalytic activity of its own. Accordingly, the selection of the components and the various physical properties of the components included in a washcoat may affect the overall catalytic performance of the catalyst. Taking the aluminum oxide component of a three-way conversion catalyst as an example, selection of the aluminum oxide component may have an effect on thermal stability, physical stability, and mass transfer between the gas to be treated and the catalytically active components of the washcoat.
Mass transfer relates to the ability of the exhaust gas to be treated to come in contact and interact with the catalytic materials of the catalyst, and is impacted by, among other variables, available surface area of a material, gas temperature, exhaust gas flow rate, and the like. As such, catalysts, and in particular three way conversion catalysts, may comprise porous materials to impart a larger surface area available for contact by the exhaust gas than would be available using a non-porous material of the same underlying size, shape and dimension. Accordingly, a need remains for further optimization of contact and improved mass transfer between a gas to be treated and a desired catalytic material.